Regenerative thermal oxidation is used for the purification of solvent-laden exhaust gas that is generated as a byproduct of various processes, such as painting for example. Typically, to carry out the oxidation process, a regenerative thermal oxidizer (RTO) 10, such as the pull-through regenerative thermal oxidizer shown in FIG. 1, is used. The RTO 10 includes two or more regenerative chambers or towers; however, for the purposes of the following discussion, the RTO 10 is shown to have three towers 20A-C. Disposed within each tower 20A-C are respective heat-exchange elements or media 30 A-C, which are formed from ceramic or any other suitable material. In addition, each of the towers 20A-C maintains an upper plenum region 40A-C that is located in the tower 20 at a point above the media 30A-C and which are operatively coupled together via a combustion chamber 50. The combustion chamber 50 maintains one or more burners 60, which are typically fueled by natural gas, propane, or other suitable fuel, for heating and oxidizing organic compounds and other pollutants in the solvent-laden air being treated or abated by the RTO 10. Alternatively the combustion chamber 50 can be heated with electric coils. In one aspect, some combustion chambers 50 have a self-sustain mode where as long as the established LEL (lower explosive limit) is maintained, no additional heat source is required (i.e., the burner/coil is off) since oxidation will be sustained without it.
Air movement through each of the towers 20A-C and the combustion chamber 50 is controlled by valve groups 70A-C that are respectively maintained proximate a lower plenum region 80A-C of the towers 20A-C, whereby the lower plenum 80A-C located at the region below the media 30A-C and above the valve groups 70A-C. The valve groups 70A-C respectively includes a set of three valves, an inlet valve 90A-C, a purge valve 92A-C, and an exhaust valve 94A-C, which controls the flow of air through the heat-exchange media 30A-C that are respectively disposed within each of the towers 20A-C. As such, the inlet valves 90A-C are operatively coupled to an inlet manifold or duct 100, which receives solvent-laden air from a pollution source 102. The purge valves 92A-C are operatively coupled to a purge manifold or duct 110 that is coupled to an exhaust stack 120. Finally, the exhaust valves 94A-C are operatively coupled to an exhaust manifold or duct 130 that is operatively coupled to an electrically powered exhaust blower 140 that expels air that is oxidized and cleaned by the RTO 10 into the exhaust stack 120. Thus, the valves 90A-C, 92A-C, and 94A-C enable each of the towers 20A-C of the RTO 10 to switch function during changes in operating cycles in accordance with any one of three primary modes: an inlet mode; a purge mode and an exhaust mode.
For example, as shown in FIG. 1, during one operating cycle of the RTO 10, the inlet valve 90A of tower 20A is opened, exhaust valve 94B of tower 20B is opened, and purge valve 92C of tower 20C is opened, while the remaining valves 92-94A, 90-92B, and 90C and 94C are closed. As such, solvent-laden air is drawn through the inlet manifold 100 and through the inlet valve 90A before entering the heat-exchange media 30A provided by the tower 20A. After entering the heat-exchange media 30A, the solvent-laden air enters the combustion chamber 50, as well as the upper part of media 30A, where it is oxidized by the heat generated by the burner 60. The heated oxidized air, which is now cleaned, passes through the heat-exchange media 30B of tower 20B, where the heat of the air is transferred to the media 30B prior to being drawn into the exhaust manifold 130, where the air is evacuated from the exhaust stack 120 by the exhaust blower 140.
Somewhat simultaneously with the evacuation of the cleaned air from the exhaust stack 120, a portion of the clean air is drawn from the exhaust stack 120 and delivered to the tower 20C via the purge duct 110 where it passes through the purge valve 92C and the media 30C before being drawn into the cleaned airflow passing through the heat-exchange media 30B and out through the exhaust duct 130. Because the media 30C and the lower plenum have accumulated pollutants and contaminates from the intake of solvent-laden air during a previous inlet operating mode, the purging of the heat exchange media 30C and lower plenum with cleaned air from the exhaust stack 120 prevents those pollutants and solvent laden air from being output at the exhaust stack 120 when the tower 20C is operated in a subsequent exhaust mode.
As previously discussed, the RTO 10 is configured so each of the towers 20A-C operates primarily in either of an inlet, exhaust, or purge mode during various operating cycles. Therefore, when the operating cycle of the RTO 10 changes at timed intervals, the tower 20A, which is currently operating in an inlet mode, switches to a purge mode; tower 20B, which is currently operating in an exhaust mode, switches to an inlet mode; and tower 20C, which is currently operating in a purge mode, switches to an exhaust mode. Moreover, in a subsequent operating cycle, the tower 20A that is operating in a purge mode switches to an exhaust mode; tower 20B, which is operating in an inlet mode, switches to a purge mode; and tower 20C, which is operating in an exhaust mode, switches to an inlet mode. As such, by alternating the operating function of each of the towers 20A-C between an inlet mode, outlet mode, and purge mode during successive operating cycles, allows the heat maintained by the heated oxidized air that is passing through the heat exchange media 30A-C associated with the exhaust mode of the tower 20A-C to be utilized in a subsequent intake mode to preheat incoming solvent-laden air. As a result, the heat generated by the burner 60 that is used to oxidize a cleaned airflow passing out of the media 30A-C is then recaptured and used to pre-heat a subsequent incoming air flow of solvent-laden air, thereby reducing the amount and extent to which the burner 60 is required to operate, thus increasing combustion efficiency. In addition, the purge cycle that is performed subsequently after one of the towers 20A-C has operated in an inlet mode ensures that spikes or severe elevations in VOCs (volatile organic compounds) or other pollutants are kept to a minimum from the tower 20A-C, when being operated in an exhaust mode after allowing solvent-laden air to enter the tower during a previous inlet mode.
However, current RTO systems 10 rarely have control over the amount of purge air that is permitted to flow into the towers 20A-C during a purge cycle, other than the selection of the diameter of the ducting forming the purge manifold 110 and in some cases a rough setting to a purge balancing damper. As such, most existing RTO systems 10 supply an excessive amount of clean air to purge the entire cross-section of the heat exchange media 30A-C, while no existing RTO system focuses on optimizing purge flow in a way to optimize performance while minimizing cost.
Furthermore, because of the operation of the RTO 10, the upper portion of the media 30A-C that is adjacent to the upper plenum region 40A-C is heated by the combustion chamber 50 to flashpoint temperatures, resulting in the abatement of accumulated pollutants therein, while the lower portion of the media 30A-C that is adjacent the lower plenum region 80A-C does not reach flashpoint temperatures, and therefore the pollutants and solvent laden air (SLA) contained therein are not abated. However, while only a portion of the ceramic media 30A-C requires purging, current RTOs operate the exhaust blower 140 to generate an airflow that is in excess of that required (and on pull-through RTO systems without a separate purge fan). As such, the exhaust blower 140 is pulling a higher volume of air than is needed, and as a result increased electrical consumption and costs are incurred. In addition to the solvent-laden air being abated within the combustion chamber 50, cooled air from the exhaust stack 120 that is used to purge the media 30 being cleaned lowers the temperature of the combustion chamber 50 from set-point/flash-point temperatures that are needed to oxidize the solvent-laden air being abated by the RTO 10. And as such, the burner 60 is required to operate for an extended period of time to reach temperatures needed to oxidize solvent-laden air in the combustion chamber 50. Thus, it would be advantageous to provide enough air to purge only the lower portion of the media 30A-C at the region of the lower plenum 80A-C that has not reached flashpoint temperatures, so as to reduce the energy consumed by the exhaust blower 140 and the burner 60, thereby reducing the overall operating costs of the RTO 10, while also reducing greenhouse emissions associated with combustion.
Therefore, there is a need for a purge air control system for a regenerative thermal oxidizer that calculates the amount of purge air needed to purge the media of an RTO tower in order to maintain destructional efficiencies at high levels. In addition, there is a need for a purge air control system for a regenerative thermal oxidizer that utilizes reduced utilities in which to purge the heat exchange media of an RTO.